1. Field of the Invention
The invention relates to a method for transmitting electrical energy, in which between two electrical units, electrical current is transmitted by means of a superconductive cable system whose two ends are each connected in a current conducting manner to one of the electrical units, and in which, further, a normally conducting cable system is arranged parallel to the superconductive cable system, wherein the function of the superconductive cable system is monitored. by a control unit.
2. Description of Related Art
Superconductive cable systems are used, for example, for transmitting electrical energy in municipal current supply networks. At sufficiently low temperatures, at which the direct current of the conductor is zero, they permit the virtual loss-free transmission of electrical energy. For example, a superconductive cable system can be used in regional transport or distribution networks (for example at a voltage of 110 kV), or for connecting transformer units within an inner city distribution network (for example at a voltage of 10 kV). A superconductive cable system can also connect the connector of a user to a municipal power network.
The superconductive cable system consists of a superconductive cable which is arranged in a cryostat for cooling. The cryostat has at least one thermally insulated pipe through which a suitable cooling agent, for example, liquid helium or liquid nitrogen, is conducted. The insulated pipe consists, for example, of two concentrically arranged metal pipes held at a distance from each other and include a vacuum insulation and other insulation materials between them. At the ends of the superconductive cable system, end closures for connection to a current supply network or electrical units are secured. These end pieces are equipped with access for feeding the cooling agent from a cooling agent reservoir. The ends of the superconductive cable system are each connected by means of power switches and other cable elements, for example, separating switches to collection rails between the transformer units or other electrical units.
The technical components of the electrical units, as well as the electrical lines between the units, are usually designed so as to be redundant so that in the case of a problem, i.e. a failure or error of one of the components, the function of the energy transmission system continues to be ensured and, for example, a current failure is avoided. In addition to the superconductive cable system, a conventional, i.e. normally conductive cable system which also is connected by means of power switches and other elements to the transformer units. The normally conductive cable system normally comprises, for example, one or more conventional cables per phase in a three phase system. During normal operation, the superconductive cable system and the normally conductive cable system can be operated so that, in case of interruption of the one cable system, the respectively other cable system can continue to operate. Alternatively, the normally conductive cable system can be switched off during normal operation.
The function of the superconductive cable system is monitored by a control or protective system. For example, the protective system measures the strength of the current or the impedance at one or both ends of the superconductive cable system. If a defective function is determined, the superconductive cable system is switched off. If the normally conductive cable system is not operated parallel to the superconductive system, the one is subsequently switched to the other. In the case of the normally conductive cable system, operated in parallel, it is a disadvantage that current always flows through the cable system and causes losses which increase the total system losses. The service life of the normally conductive cable system is decreased by being permanently switched on.
In the case of a normally conductive cable system, there is the disadvantage that the current supply of the network is not ensured after the superconductive cable system has been switched off and before the normally conductive cable system is switched on. In Jun Yang et al, Power and Energy Society General Meeting —Conversion and Delivery of Electrical Energy in the 21st Century, 2008 IEEE, 20 Jul. 2008, p. 1-5, a high temperature superconductive (HTS) cable system is described, which includes an HTS cable and a parallel conventional cable. A conventional cable is switched off, in the case of failure of the HTS cable, and the HTS system is switched off immediately subsequently.